Transmembrane transport is a critical cellular function. Because practitioners have recognized the importance of transmembrane transport to many areas of medical and biological science, including drug therapy and gene transfer, there have been significant research efforts directed to the understanding and application of such processes. Thus, for example, transmembrane delivery of nucleic acids has been attempted through the use of protein carriers, antibody carriers, liposomal delivery systems, electroporation, direct injection, cell fusion, viral carriers, osmotic shock, and calcium-phosphate mediated transformation. However, many of those techniques are limited both by the types of cells in which transmembrane transport occurs and by the conditions required for successful transmembrane transport of exogenous molecules. Furthermore, many of these techniques are limited by the type and size of the exogenous molecule that can be transported across the cell membrane without loss of bioactivity.
One mechanism for transmembrane transport of exogenous molecules having wide applicability is receptor-mediated endocytosis. Advantageously, receptor-mediated endocytosis occurs both in vivo and in vitro. Receptor-mediated endocytosis involves the movement of ligands bound to membrane receptors into the interior of an area bounded by the membrane through invagination of the membrane. The process is initiated or activated by the binding of a receptor-specific ligand to the receptor. Many receptor-mediated endocytotic systems have been characterized, including those resulting in internalization of galactose, mannose, mannose 6-phosphate, transferrin, asialoglycoprotein, folate, transcobalamin (vitamin B12), α-2 macroglobulins, insulin, and other peptide growth factors such as epidermal growth factor (EGF).
Receptor mediated endocytosis has been utilized for delivering exogenous molecules such as proteins and nucleic acids to cells. Generally, a specific ligand is chemically conjugated by covalent, ionic, or hydrogen bonding to an exogenous molecule of interest, forming a conjugate molecule having a moiety (the ligand portion) that is still recognized in the conjugate by a target receptor. Using this technique the phototoxic protein psoralen has been conjugated to insulin and internalized by the insulin receptor endocytotic pathway (Gasparro, Biochem. Biophys. Res. Comm. 141(2), pp. 502-509, Dec. 15, 1986); the hepatocyte specific receptor for galactose terminal asialoglycoproteins has been utilized for the hepatocyte-specific transmembrane delivery of asialoorosomucoid-poly-L-lysine non-covalently complexed to a plasmid (Wu, G. Y., J. Biol. Chem., 262(10), pp. 4429-4432, 1987); the cell receptor for EGF has been utilized to deliver polynucleotides covalently linked to EGF to the cell interior (Myers, European Patent Application 86810614.7, published Jun. 6, 1988); the intestinally situated cellular receptor for the organometallic vitamin B12-intrinsic factor complex has been used to mediate delivery of a drug, a hormone, a bioactive peptide and an immunogen complexed with vitamin B12 to the circulatory system after oral administration (Russell-Jones et al., European patent Application 86307849.9, published Apr. 29, 1987); the mannose-6-phosphate receptor has been used to deliver low density lipoproteins to cells (Murray, G. J. and Neville, D. M., Jr., J. Biol. Chem., Vol. 255 (24), pp. 1194-11948, 1980); the cholera toxin binding subunit receptor has been used to deliver insulin to cells lacking insulin receptors (Roth and Maddox, J. Cell. Phys. Vol. 115, p. 151, 1983); and the human chorionic gonadotropin receptor has been employed to deliver a ricin a-chain coupled to HCG to cells with the appropriate HCG receptor (Oeltmann and Heath, J. Biol. Chem., vol. 254, p. 1028 (1979)).
In one embodiment the present invention involves the transmembrane transport of a radionuclide-based imaging agent across a membrane having receptors for a vitamin, or a vitamin receptor binding derivative or analog thereof. A cell membrane bearing vitamin receptors, or receptors for vitamin derivatives or analogs, is contacted with a vitamin-imaging agent conjugate for a time sufficient to initiate and permit transmembrane transport of the conjugate, and the biodistribution of the vitamin-imaging agent conjugate in the animal is monitored. In another embodiment, the vitamin/vitamin derivative or analog targeting moiety simply binds to a cell surface vitamin receptor to concentrate the chelated radionuclide on the cell surface.
The invention takes advantage of (1) the location of vitamin receptors and (2) the associated receptor-mediated endocytic processes. For example, the invention takes advantage of the unique expression, overexpression, or preferential expression of vitamin receptors, transporters, or other surface-presented proteins that specifically bind vitamins, or derivatives or analogs thereof, on tumor cells or other cell types which overexpress such receptors. Accordingly, the invention can be used to detect cells, such as tumor cells or other cell types, which overexpress vitamin receptors, or receptors for vitamin derivatives or analogs, by taking advantage of the receptor-mediated endocytic processes that occur when such cells are contacted with the vitamin-imaging agent conjugate.
Vitamin receptors, such as the high-affinity folate receptor (FR) is expressed at high levels, for example, on cancer cells. Epithelial cancers of the ovary, mammary gland, colon, lung, nose, throat, and brain have all been reported to express elevated levels of the FR. In fact, greater than 90% of all human ovarian tumors are known to express large amounts of this receptor. Thus, the present invention can be used for the diagnostic imaging of a variety of tumor types, and of other cell types involved in disease states.
Radionuclide chelators complexed to ligands have been used as non-invasive probes for diagnostic imaging purposes. For example, vasoactive intestinal peptide, somatostatin analogs, and monoclonal antibodies have been used as ligands to localize radionuclides to cells, such as tumor cells. Monoclonal antibodies, and various fragments thereof, initially received the most attention because it was believed that precise tumor-specific targeting might be achieved using monoclonal antibodies as targeting ligands. Unfortunately, this approach was problematic because i) antibodies have prolonged circulation times due to their large size which is unfavorable for imaging purposes, ii) antibodies are expensive to produce, iii) antibodies can be immunogenic, and, accordingly, must be humanized when multiple doses are used, and iv) tumor to non-target tissue ratios (T/NT) of antibody-linked radionuclides are sub-optimal. Thus, the focus has recently been directed to the use of smaller tumor-specific ligands that do not have such limitations.
Vitamins, such as folic acid, have been used for the targeting of imaging agents to tumor cells, and are advantageous because of their small size. The first folic acid-based targeting complex described for in vivo tumor imaging was a histamine derivative containing 125Iodine. This complex was not considered a relevant clinical candidate because of the long-lived 125I radionuclide component. Subsequently, a deferoxamine-folate conjugate for tumor imaging was developed (deferoxamine chelates 67Ga, a gamma-emitting radionuclide that has a 78 hour half-life). Hepatobiliary clearance was noted with this conjugate and, thus, preclinical development was stopped due to anticipated problems in accurately imaging regio-abdominal locations. This obstacle was overcome, however, by replacing the deferoxamine chelator with diethylenetriamine pentaacetic acid (DTPA), an efficient chelator of 111In (68 hour half life). The primary route of elimination of 111In-DTPA-folate was confirmed to be through the kidneys.
More recently, 99mTc has been adopted as the preferred radionuclide for diagnostic imaging, because i) the radionuclide is easily obtained from commercially available 99Mo—99mTc generators, ii) the cost of producing large amounts of 99mTc is insignificant compared to the cost of producing 111In, and iii) 99mTc has a much shorter (6 hour) half life, which allows higher radionuclide doses to be administered, yielding higher resolution images without the risk of hazardous radiation exposure to vital organs.
Several folate-based 99mTc conjugates have been developed. For example, folate conjugates of 99mTc-6-hydrazinonicotinamido-hydrazido (HYNIC; Guo, et al., J. Nucl. Med., 40(9): 1563-1569 (1999)), 99mTc˜12-amino-3,3,9,9-tetramethyl-5-oxa-4,8 diaza-2,10-dodecanedinoe dioxime (OXA) (Linder, et al., Soc. Nucl. Med., Proc. 47th Annual Meeting, 2000, 41(5): 119P), 99mTc-ethylenedicysteine (Ilgan, et al., Cancer Biother. & Radiopharm., 13(6): 427-435 (1998)), and 99mTc-DTPA˜folate (Mathias, et al., Bioconjug. Chem., 11(2): 253-257 (2000)) have shown promising in vivo tumor uptake qualities. However, there is a need for alternative vitamin-based 99mTc conjugates, or vitamin-based conjugates employing other radionuclides, that exhibit optimal tumor to non-target tissue ratios (T/NT) and are eliminated through the kidneys. Such vitamin-based conjugates should be suitable for clinical development as tumor imaging agents, and for the diagnosis of other disease states.
In one embodiment is provided a compound of the formula
wherein V is a vitamin, or a vitamin receptor binding derivative or analog thereof, L is a divalent linker, R is a side chain of an amino acid of the formula H2NCHRCOOH, M is a cation of a radionuclide, n is 1 or 0, and k is 1 or 0. The vitamin is a substrate for receptor-mediated transmembrane transport in vivo.
In another embodiment is provided a composition for diagnostic imaging comprising a compound of the formula
wherein V is a vitamin, or a vitamin receptor binding derivative or analog thereof, L is a divalent linker, R is a side chain of an amino acid of the formula H2NCHRCOOH, M is a cation of a radionuclide, n is 1 or 0, and a pharmaceutically acceptable carrier therefor. The vitamin is a substrate for receptor-mediated transmembrane transport in vivo.
In yet another embodiment a method is provided of imaging a population of cells in an animal, wherein the cells are characterized by a vitamin receptor on the surface of the cells. The method comprises the steps of administering to the animal an effective amount of a composition comprising a compound of the formula
wherein V is a vitamin, or a receptor binding derivative or analog thereof, specific for the cell surface vitamin receptor, L is a divalent linker, R is a side chain of an amino acid of the formula H2NCHRCOOH, M is a cation of a radionuclide, n is 1 or 0, and a pharmaceutically acceptable carrier therefor, and monitoring the biodistribution of the compound in the animal.
In another embodiment a compound is provided of the formula
wherein V is a vitamin that is a substrate for receptor-mediated transmembrane transport in vivo, or a vitamin receptor binding derivative or analog thereof, L is a divalent linker, R is a side chain of an amino acid of the formula H2NCHRCOOH, M is a cation of a radionuclide, n is 1 or 0, and k is 1 or 0.
In still another embodiment, a composition for diagnostic imaging is provided comprising a compound of the formula
wherein V is a vitamin that is a substrate for receptor-mediated transmembrane transport in vivo, or a vitamin receptor binding derivative or analog thereof, L is a divalent linker, R is a side chain of an amino acid of the formula H2NCHRCOOH, M is a cation of a radionuclide, n is 1 or 0, and a pharmaceutically acceptable carrier therefor.
In yet another embodiment, a method of imaging a population of cells in an animal is provided wherein the cells are characterized by a vitamin receptor on the surface of the cells. The method comprises the steps of administering to the animal an effective amount of a composition comprising a compound of the formula
wherein V is the vitamin, or a receptor binding derivative or analog thereof, specific for the cell surface vitamin receptor, L is a divalent linker, R is a side chain of an amino acid of the formula H2NCHRCOOH, M is a cation of a radionuclide, n is 1 or 0, and a pharmaceutically acceptable carrier therefor, and monitoring the biodistribution of the compound in the animal.
In any of these embodiments, V in the compound can be, for example, a vitamin selected from the group consisting of folate, riboflavin, thiamine, vitamin B12, and biotin, or a derivative or analog thereof. In any of these embodiments, the radionuclide in the compound can be selected, for example, from the group consisting of radioisotopes of gallium, indium, copper, technetium, and rhenium.